Link Budgets

A link budget is a calculation of the amount of power received at a given
receiver based on the output power from a given transmitter. The link budget
accounts for all of the gains and losses that a radio wave experiences
along the path from transmitter to receiver. For a given transmitter power,
we determine the maximum path loss that the signal can experience in
order for the signal to be recoverable at the receiver. Given that the base
station must be able to “hear” the mobile and the mobile must be able to
“hear” the base station, we need to perform the calculation in both directions—
from mobile to base station and from base station to mobile. We
determine the maximum allowable path loss in each direction and the
lesser of the two corresponds to the coverage limit for the cell and service in
question. For example, if the maximum allowable path loss in the uplink is,
say 130 dB, and the maximum allowable path loss in the downlink is, say
135 dB, then we should not exceed 130 dB, and we are said to be uplink limited.
The link budget needs to include a margin (that is, a buffer) to enable
fading of the signal. In other words, we design the system such that service
will still be supported even in the case of where the signal fades significantly.
The greater the fade margin, the greater the reliability of the service.
Moreover, because a Wideband CDMA (W-CDMA) system is interference
limited, we also need to include an interference margin. As described later
in this chapter, the size of that margin is load dependent.

As mentioned, the effective cell coverage is dependent upon the service to
be provided. One reason for this is the fact that the higher the spreading
factor (corresponding to a lower data rate), the higher the processing gain,
and the lower the spreading factor, the lower the processing gain. Because
the processing gain is one of the gains that needs to be included in a link
budget, it follows that the lower the processing gain, the lower the maximum
allowable path loss and the smaller the effective radius of the cell.
From a pure radio propagation point of view, we will generally find that
coverage is uplink limited, if for no other reason than that the output power
of the base station is far greater than that of the mobile. As we shall see,
however, cell loading also impacts coverage, so the consideration of cell load
must be considered in coverage analysis.

Tables 12-1, 12-2, and 12-3 provide example uplink link budgets for three
WCDMA services—speech service at 12.2 Kbps outdoors, data service at
128 Kbps indoors, and data service at 384 Kbps indoors.

NOTE: Thermal noise density  kT, where k  Boltzmann’s constant
and T is temperature in Kelvin. T is usually assumed to be 293K.
NOTE2: The log normal fade margin is a design value and depends
upon the required level of signal reliability over the cell area. 7.5 dB
corresponds to 93.4% coverage probability.
In Table 12-1, we have a link budget that would apply to outdoor (nonvehicular)
speech service. The user device has a nominal power output of
0.125 W (21 dBm).Thus, it is likely to be a Power Class 4 device (max power
of 21 dBm, 2 dB) or a Power Class 3 device (max power of 24 dBm,1/
3
dB).We assume that there is no antenna gain for the device, and we assume
a 3 dB body loss as the device is likely to be close to user, and the signal will
have to pass through the user.

At the receiving side, we assume a receiver noise figure of 5 dB and an
interference margin of 4 dB. The interference margin accounts for the fact
that there will be interference at the base station caused by multiple users.
The greater the number of users, the greater the interference and the
greater the required interference margin. Also at the receiving side, we
specify the processing gain and the Eb/No required for the service. As we will
describe shortly, the required Eb/No can vary according to the service in
question.
If we include a typical antenna gain value for the base station antenna
and typical losses for cables and connectors, then simple addition gives the
maximum path loss. In reality, however, we need to add some additional
considerations to account for real-world situations.

First, we need to add a fast fading margin. This is a buffer to enable for
the mobile to adjust power according to closed loop power control. If the link
budget in Table 12-1 were prepared for a mobile moving at a fast speed
(such as 60 mph), then closed loop power control would be unlikely to be fast
enough to change the transmitted power in response to the rapid changes
in pass loss as the mobile moves. Thus, for a high-speed vehicular service,
one would set the fast fading margin to zero.

We also need to add a log-normal fading margin, with a value that is
determined by the desired cell area (or cell edge) coverage reliability. The
higher the desired coverage reliability, the higher the log normal fading
margin. Finally, we can add a gain that results from soft handover. Basically,
if a subscriber is being covered by more than one cell and is in a softhandover
situation, then the signal from the handset is being received by
two base stations (or perhaps by two cells at the same base station site).

This is the equivalent to an extra level of receiver diversity and offers a similar
gain.
In Table 12-2, we have a link budget that would apply to an indoor data
service at 128 Kbps. In this case, the service is assumed to be provided by a
base station located outside of the building in question. The user device has
a nominal power output of 0.25 W (24 dBm). Thus, it is likely to be a Power
Class 3 device (max power of 24 dBm,1/
3 dB) or a Power Class 2 device
(max power of 27 dBm, 1/
3 dB). We assume that there is no antenna
gain for the device.We further assume that, unlike the case for a speech service,
the device is less likely to be very close to the user (that is, not against
the user’s head). Therefore, we do not allow for any body loss.

At the receiving side, many of the parameters are the same as for the
example of Table 12-1. The required Eb/No in this case, however, is 2 dB, and
the processing gain is lower (due to the higher data rate). The other margins,
gains, and losses are the same as for Table 12-1, with the exception of
the building penetration loss, which we assume to be 15 dB. This figure is
highly dependent on the area to be covered. In a dense urban environment,
for example, the building penetration loss could be significantly higher.
In Table 12-3, we have a link budget that would apply to an indoor data
service at 384 Kbps.We assume that the service is to be provided by a base
station located outside of the building in question. The user device has a
nominal power output of 0.25 W (24 dBm). Given that this is likely to be a
specialized data device with an external antenna, we assume an antenna
gain of 2 dBi for the device.We also assume that there is no body loss.
At the receiving side, many of the parameters are the same as for the
example of Table 12-2. The required Eb/No in this case, however, is 1 dB, and
the processing gain is lower (due to the higher data rate). The other margins,
gains, and losses are the same as for Table 12-2.We assume the same
building penetration loss as in Table 12-2 because we are assuming that the
service is provided from a base station outside of the building. If we were to
assume an in-building base station, then the penetration loss would be
much lower—just enough to accommodate for losses in internal walls
within the building.

In reviewing the three example link budgets, we see that the maximum
allowed path loss decreases as the required data rate increases. Thus, the
higher the data rate to be offered over a given area, the greater the required
density of base stations.
There is not an exact “apples-to-apples” comparison between the different
services in our example link budgets as we have made different
assumptions regarding mobile output power, antenna gains, and building penetration losses. If, for comparison purposes, we were to assume that
these quantities were the same in each scenario, then we would have a clear
picture of how the cell coverage reduces as the data rate increases (all other
things being equal). The reduction in cell coverage is due to the reduced processing
gain. It is noticeable, however, that while there is a reduced processing
gain for higher data rates, this is somewhat counterbalanced by a
lower Eb/No requirement for higher data rates.

The required Eb/No is dependent on many factors including the mobile
speed, data rate, and multipath profile. Why should the Eb/No decrease as
the data rate increases? The answer is the fact that higher bit rates mean
greater power output from the mobile. There is greater output power for
both the Dedicated Physical Control Channel (DPCCH) and the Dedicated
Physical Data Channel (DPDCH) as the data rate increases. The pilot symbols
on the DPCCH are used for channel estimation and received Signal-to-
Interface Ratio (SIR) estimation. As the DPCCH power increases, the better
the channel estimation, which means that a lower Eb/No can be accommodated.
Of course, as the data rate increases, the DPDCH power also
increases and, in fact, the relative power of the DPCCH versus the DPDCH
decreases. In other words, as the data rate increases, a greater proportion of
the total power is allocated to DPDCH rather than DPCCH. But the fact
that the overall power increases with increasing data rate means that the
total DPCCH power increases (albeit not as much as the DPDCH power). It
is the absolute DPCCH power that is important in channel estimation, and
because the absolute DPCCH power increases, the required Eb/No decreases.